A method has now been developed for conversion of letterpress units to offset printing units by removing the letterpress cylinders, placing an auxiliary frame for supporting offset printing cylinders inside the main frame, and then installing the offset printing cylinders inside the auxiliary frame.
A perfecting or multicolor offset printing press invented by the present inventor is disclosed in U.S Pat. No. 3,329,086. That patent shows the combination of a frame, a pair of plate cylinders laterally spaced in the frame for mounting printing plates thereon with a pair of blanket cylinders in rolling contact with their respective plate cylinders and an impression cylinder centered with respect to the cusp formed by the blanket cylinders. The blanket cylinders can be moved through a short radius arc between a first position in which the blanket cylinders make contact with their respective plate cylinders and with each other for blanket-to-blanket printing and a second position in which in the blanket cylinders make contact with their respective plate cylinders and with the impression cylinder for blanket-to-impression printing. The first position can be used for black printing on both sides of a web or single color perfecting when there is a half deck for providing an additional blanket cylinder in rolling contact with the impression cylinder. The second position can be used for applying two colors to the same side of a web, or three colors if there is a half deck.
To convert a black letterpress unit to the perfecting or multicolor offset press shown in the inventor's prior patent by the conversion process described requires substantial reworking of the letterpress unit including installing an auxiliary frame which is capable or supporting at least both ends of both blanket cylinders and the impression cylinder. This is necessary because of the distances through which the blanket cylinders are swung to go between the different modes of operation. In addition, if a web breaks while it is in the cusp between the blanket cylinders and the impression cylinder, the resultant web jamb can be very difficult to clear.